1. Field of the Invention
The present invention relates to an electrochromic device, and more particularly, to an electrochromic device having improved color properties and method of manufacturing the same.
2. Description of the Related Art
In general, a reversible color change caused by an application of an electric field is referred to as “electrochromism,” and a material having optical properties that are able to reversibly change colors in response to an electrochemical redox reaction is referred to as an “electrochromic material.” That is, the electrochromic material is characterized in that the electrochromic material is in an uncolored state in an absence of an external electric field and the electrochromic material is then colored by an application of an external electrical field, or, conversely, the electrochromic material is in a colored state in the absence of an external electric field and then the color of the electrochromic material is made to disappear by the application of the electric field. Examples of such electrochromic materials include, but are not limited to, metal oxides, for example, tungsten oxide and molybdenum oxide, and organic compounds, for example, pyridine compounds, aminoquinone compounds and viologen.
An electrochromic device using such an electrochromic principle has superior reflectivity, outstanding flexibility and portability and is lightweight, and therefore such electrochromic devices are expected to be used in various flat panel displays. In particular, the electrochromic device may be applied to e-paper, which is under thorough study as an electronic medium that may replace paper, and thus is receiving increasing attention.
FIG. 1A is a schematic cross-sectional view illustrating a conventional electrochromic device of the prior art. The electrochromic device 1 includes an upper transparent electrode 10 coated with a transparent semiconductor material (e.g., tin oxide (“TiO2”)) and an electrochromic material 20, and a lower electrode 50 coated with a counter material 40 (e.g., antimony-doped tin oxide (“ATO”)), for a more efficient electrochemical reaction, and with a reflective material 30 for reflecting light.
The electrochromic material 20 is in a transparent state in an absence of an electric field, and thus admits light therethrough, however while the electrochromic material 20 is oxidized or reduced by an application of an electric field, the electrochromic material 20 displays a predetermined color. That is, when no electric field is applied, the electrochromic material 20 is in a transparent state, and thus there is no wavelength of light that is absorbed by the electrochromic material 20. Accordingly, all wavelengths of incident light are allowed to pass through the upper transparent substrate 10 and are then reflected from the lower reflective layer 30, thus emitting light again to the upper surface of the device. Consequently, an observer positioned at a front of the display device may see a white color (FIG. 1B).
However, when an electric field is applied, the electrochromic material 20 of the electrochromic device 1 displays a predetermined color in response to the oxidation or reduction of electrons therein, and thereby all wavelengths of light, other than the predetermined color, are absorbed. As illustrated in FIG. 1C, only the predetermined color of the electrochromic material 20 is allowed to pass through the upper transparent substrate 10, and light having the other wavelengths is absorbed by the electrochromic material 20. Ultimately, because light of the predetermined color is exclusively emitted again to the upper surface through the lower reflective substrate, an observer may see such a predetermined color.
Further, the electrochromic device 1 may be formed into a color display device having red, green and blue elements as a single unit element, similar to general display devices. FIGS. 2A through 2F illustrate a color display principle of a conventional display device of the prior art using three color elements as a single unit element.
In order to display a white color in such a conventional electrochromic device, an electric field is eliminated from all of color display elements (FIG. 2A). As such, since the electrochromic materials of the color display elements are in a transparent state, white light is transmitted through all of the color display elements and is then reflected from the reflective substrate. Accordingly, an observer may see a white color.
In order to display a red color, an electric field is applied to a red display element but the electric field is not applied to the other color elements (i.e., green and blue). Thereby, red light is allowed to pass through the red display element and is then reflected, thus displaying a red color. Further, the other color elements display a white color, and consequently an observer recognizes a red color (FIG. 2B). Similarly, a green color or a blue color may be displayed by applying an electric field to the corresponding display element (FIG. 2C, 2D).
In particular, in a case of displaying a black color, an electric field is applied to all of the color display elements, such that respective display elements exhibit a red color, a green color and a blue color. At this time, because light is reflected in a smaller amount than in the case of displaying a white color, an observer recognizes a black color (FIG. 2E).
In this way, in the case where the electrochromic device displays a black color, the electrochromic device exhibits a phenomenon that is opposite to that of a typical light-emitting display device. That is, when all of the red, green and blue display elements are driven, to thereby emit light, the typical light-emitting display device displays a white color, however the conventional electrochromic device of the prior art displays a black color.
The electrochromic device accords to the principle in which light is emitted in a smaller amount than in the case of a white color, to thereby make an observer recognize a black color. In this case, however, since light is incident on the eyes of an observer in reality, a visibility thereof is inevitably decreased.
As illustrated in FIG. 2F, good visibility may be assured when light is not emitted through any portion of a surface area of the electrochromic device. However, in the case where the conventional electrochromic device displays a black color, it is difficult to realize a state in which light is not emitted through the surface area of the electrochromic device due to the structural properties thereof.